The present invention relates generally to a measurement device that characterizes the three dimensional (3D) load displacement response of a joint. Specifically, the present invention relates to an apparatus and method for determining load displacement characteristics of an anatomical joint in a clinical or experimental environment, including primary and coupled motions while negating the influence of gravitational forces.
Clinical assessment of joint pathologies, including those involving soft tissues or other ligament injuries, is primarily subjective. The clinician relies on the manual manipulation of the joint. During clinical examination, the clinician obtains a qualitative and subjective “feel” for the amount of laxity or stiffness in the involved joint as compared to the clinician's previous intuition regarding what the normal joint should feel like and/or compared with the contralateral joint should it be uninjured. This clinical examination is then supplemented by the clinical history of the patient and by radiological evaluations. Moreover, assessment of the effectiveness of treatment be it conservative (e.g., rehabilitation) or surgical is subjective and relies on feedback from the patient.
Knee rotations and coupled motions as measured during clinical examination of the knee (pivot shift phenomenon) are an important predictor of clinical outcome and a potential risk factor for osteoarthritis following ligament injuries, such as the ACL. However, conventional knee arthrometers are unidirectional, measuring the translation or rotation only in the direction of the applied force or torque, respectively. They were not designed to characterize knee rotations and coupled motions occurring in response to multiplanar torques, such as what occurs during the pivot shift exam. Therefore, a need remains for a standardized, quantitative, objective, clinically-usable method to assess rotations and coupled motions. Previous devices that measured rotations and coupled motions were cumbersome or relied on complicated, time-consuming methods making them unsuitable for clinical use. Robotic technology has been used extensively in vitro to explore the rotations and coupled motions occurring in response to combined valgus and internal rotation torques, thus simulating the pivot shift exam. Unfortunately, such data has not translated to development of a useful clinical tool. Knee laxity has also been assessed using MR-compatible devices to negate soft tissue artifact; however, the approach is time-consuming and costly, which limits its clinical utility.
Further, conventional arthrometers do not have the capability of balancing each axis of the system resulting in interference with free movement along each axis of the device because the weight of the various segments forming the device act on each axis and generate undesired forces and moments across the anatomical joint due to gravity. Consequently, there is a need for a quantitative, reliable technique to assess the 3D load displacement response of a joint for the purposes of contributing to the clinical management of joint pathologies.